The present invention relates to a semiconductor device having a drive circuit element and an output transistor.
Hitherto, in power semiconductor devices manufactured by planar technology, for instance, as shown in FIG. 1, there is known a device which includes drive circuit elements 1 to 3 formed in the surface area of an n.sup.- -type layer 4 formed on a p-type semiconductor substrate; and an output pnp transistor 5 having the n.sup.- -type layer 4 as a base and the semiconductor substrate as a collector. The drive circuit element 1 is the p-channel MOS transistor formed in the surface area of the n.sup.- -type layer 4. The drive circuit element 2 is the n-channel MOS transistor formed in a p-well formed in the surface region of the n.sup.- -type layer 4. The drive circuit element 3 is the npn transistor having the n.sup.- -type layer 4 as a collector. These drive circuit elements 1 to 3 are isolated from the output transistor 5 by a p.sup.+ -type isolation region 6.
In the semiconductor device in which the output transistor is constituted by the bipolar transistor as described above, an excellent saturation voltage characteristic is obtained; however, there are drawbacks such that it requires a large amount of drive power and its switching speed is slow. To eliminate such drawbacks, a method whereby the output transistor is constituted by a power MOS transistor has been considered, but in this case, the ON-resistance of the power MOS transistor is large; therefore, this necessitates an enlargement of the chip size to handle a current of the same magnitude as that of the bipolar transistor. Further in the case where an n-channel power MOS transistor having a larger current capacity than a p-channel MOS transistor is used, it is required to substitute an n.sup.+ -type layer for the p.sup.+ -type layer in the semiconductor substrate, making its structure complicated.